Gap Junction Subunits Research Articles

Linear gap and tight junctional assemblies between capillary endothelial cells in the eel rete mirabile.

Interendothelial tight junctions and gap junctions have been described in large blood vessels and in cultures of endothelium derived from large blood vessels. Transfer of microinjected small-molecular weight tracers between adjacent endothelial cells also has been demonstrated indicating the presence of gap junctional interendothelial communication. Similar transfer of tracers is evident between microvessel endothelial cells in culture and in microvessels in situ. However, gap junctions have not been detectable by electron microscopy of intact capillary systems. This may be due to limited sampling available in diffuse capillary systems and a small area of overlap between adjacent endothelial membranes. Thin slices of the parallel, tightly packed capillary bed of the eel rete mirabile were cryofixed and prepared for conventional TEM by freeze-substitution. Other samples were freeze-fractured and replicated for examination of endothelial junctional components. A novel tight-gap junctional complex between rete capillary endothelial cells is described. In freeze-fracture replicas of the membrane P face, rows of gap junction subunits are flanked on either side by linear depressions representing grooves previously occupied by tight junctional strands that partition to the E face. In thin sections, the junctions appear in profile as short lengths of closely apposed membranes characteristic of gap junctions. The tight junctional components imply a barrier to paracellular transport across the capillary wall between the endothelial cells. The gap junctional component may provide a mechanism for communication between endothelial cells along the length of the vessel wall.

Immunocytochemical analysis of the expression of gap junction protein connexin 43 in the rat ovary.

The present immunocytochemical study examines in the rat ovary the pattern of expression of connexin 43 (Cx43), a subunit of gap junctions. Using a well-characterized specific antiserum against rat Cx43, immunoreactivity was not detected in the fetal ovary, i.e., prior to follicular formation. However, in the ovary of 20-day-old, 35-day-old, and adult rats, strong Cx43-immunoreactivity was associated with the cell borders of the follicular epithelium/granulosa cells of all developmental stages (primordial follicles, preantral and antral secondary follicles). In general, immunoreactivity of the granulosa cells of large antral follicles appeared more intense than the one of smaller follicles. Staining was also seen in oocytes (cytoplasmic staining). Theca cells of large antral follicles, but not of small follicles were immunoreactive. Immunoreactive interstitial cells were not seen in ovaries of 20- and 35-day-old animals, but staining in these cells was present in adult rats. In large follicles with signs of atresia, granulosa cells lacked Cx43-immunoreactivity, whereas Cx43-immunoreactivity in their theca interna strikingly increased. Corpora lutea in the cyclic adult rats were heterogeneously stained, with either no detectable immunoreactivity, staining of cell borders of most luteal cells, or with conspicuous staining of only a few cells. In the pregnant animals on gestation days (GD) 12, 14, and 17, all luteal cells stained strongly for Cx43 at the cell surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromosomal assignments of mouse connexin genes, coding for gap junctional proteins, by somatic cell hybridization

The connexin genes Cx31 and Cx45 coding for proteins of gap junctional subunits have been assigned to mouse chromosomes 4 and 11 by Southern blot hybridization of specific gene probes to DNA from mouse x Chinese hamster somatic cell hybrids. In addition, our results confirm the recent assignment of mouse connexin genes Cx26, Cx32, Cx37, Cx40, Cx43, and Cx46 to mouse chromosomes 14, X, 4, 3, 10, and 14, respectively, by analysis of interspecific backcrosses and by somatic cell hybridization. Our assignment of the Cx31 gene to mouse chromosome 4 locates the fourth connexin gene on this mouse chromosome to which the genes for Cx31.1, Cx37, and Cx30.3 have previously been assigned. Interestingly three of them (coding for Cx31, Cx31.1, and Cx30.3) are preferentially expressed in skin. Possibly some of the connexin genes clustered on mouse chromosome 4 may be regulated coordinately.

Refined localization of human connexin32 gene locus, GJB1, to Xq13.1

Connexins are the peptide subunits of gap junctions that interconnect cells to allow the direct, intercellular transfer of small molecules. Recently, the human connexin32 gene (locus designation GJB1) has been regionally mapped by three other laboratories to Xp11–q13, Xcen-q22, and Xp11–q22. The smallest region of overlap from these studies is Xcen-q13. By using a series of somatic cell hybrid mapping panels and a rat connexin32 cDNA probe, we have localized the human GJB1 locus to a much smaller region in proximal Xq13.1, in interval 8, as described by Lafrenière et al. (8).

Differential regulation of the levels of three gap junction mRNAs in Xenopus embryos.

Xenopus mRNAs that potentially encode gap junction proteins in the oocyte and early embryo have been identified by low-stringency screening of cDNA libraries with cloned mammalian gap junction cDNAs. The levels of these mRNAs show strikingly different temporal regulation and tissue distribution. Using a nomenclature designed to stress important structural similarities of distinct gap junction gene products, the deduced polypeptides have been designated the Xenopus alpha 1 and alpha 2 gap junction proteins. The alpha 2 gap junction mRNA is a maternal transcript that disappears by the late gastrula stage. It is not detected in any organ of the adult except the ovary, and resides primarily, if not exclusively, in the oocytes and early embryos. The alpha 1 gap junction mRNA appears during organogenesis, and is detected in RNA from a wide variety of organs. It is also found in full-grown oocytes, but is rapidly degraded upon oocyte maturation, both in vivo and in vitro. The alpha 1 and alpha 2 mRNAs encode proteins with different degrees of amino acid sequence similarity to the predominant gap junction subunit of the mammalian heart (connexin 43). Together with our earlier report of a mid-embryonic (beta 1) gap junction mRNA, the results suggest that intercellular communication during oocyte growth and postfertilization development is a complex phenomenon involving the coordinated regulation of several genes.

Characterization of a connexin homologue in cultured soybean cells and diverse plant organs

Antibodies were prepared against ratliver connexin (27-kDa polypeptide subunit of cell gap junctions found between contacting animal cells) and a putative soybean (Glycine max (L.) Merr.) connexin (29-kDa polypeptide) previously isolated from cultured soybean root cells (SB-1 cell line). The antibodies were utilized to examine the intracellular localization of soybean connexin in these cultured soybean cells and to probe for the presence of a soybean-type connexin in petals, fruits, and leaves from a variety of plants. As judged by specific reactivity on immunoblots, both antibodies against the 27-kDa polypeptide (ratliver connexin) and against the 29-kDa polypeptide (operationally termed soybean connexin) were utilized to demonstrate immunological relatedness of the 27-kDa (rat liver) and the 29-kDa (soybean) polypeptide. Immunofluorescent localization of the putative soybean connexin in cultured soybean cells utilizing these probes demonstrated a peripherally localized punctate pattern of labeling at areas of contact between cells. Use of antibody to the soybean connexin as a probe on immunoblots of extracts from petals, fruits and leaves demonstrated that the soybean-type connexin is present in a large number of different plants.

Glucose alters configuration of gap junctions between pancreatic islet cells.

In rat pancreatic islets, gap junctional subunits (GJS) occur under two different configurations, namely in linear single strands and in polygonal particle aggregates. The present freeze-fracture study demonstrates that GJS can rapidly (dis)assemble into one of these membrane specializations without changes in their total number. Isolation of the pancreatic gland and its perfusion at 2.8 mM glucose is accompanied by a decrease in polygonally packed GJS from 46 to 16%. A rise in medium glucose concentration is followed, within 10 min, by a dose-dependent increase in the percent polygonal particles. This glucose effect on gap junction configuration is calcium dependent and reversible upon glucose removal; it is still entirely detectable when protein synthesis is blocked by cycloheximide. These results indicate that islet gap junctions are dynamic structures that rapidly adjust their configuration to extracellular regulators of beta-cell function. In the light of previous observations, it is suggested that this rapid (dis)assembly of gap junctional structures be considered as a component in the ionic and metabolic coupling between insulin-containing beta-cells of the pancreas.

Cell biology and protein composition of cardiac gap junctions

The gap junctions that electrically couple mammalian myocardial cells have high (12,000-17,000/micron2)surface densities of channel-containing elements (connexons), undulating surfaces, and approximately hexagonally arrayed connexons disposed in small domains rotated with respect to one another. Optical diffraction combined with image processing of negatively stained isolated rabbit heart gap junctions shows six protein subunits surrounding the cell-to-cell channel and suggestive (but not conclusive) evidence for protein connections between connexons. Biochemical studies indicate that the six identical relative molecular weight (Mr) 47,000 subunits of mammalian cardiac gap junctions differ from liver gap junctions in the presence of a covalently bound Mr 17,500 cytoplasmic surface component that can be visualized electron microscopically in thin-sectioned and freeze-etched hearts. The cytoplasmic surface component is susceptible to cleavage by an alkaline serine protease released from mast cell granules by high ionic strength solutions (0.6 M KI) used to extract myofibrils during gap junction purification. Interlocking of connexons from coupled cells in the gap involves hydrogen bonding between protein subunits of the connexons.

Gap junctions in the stria vascularis and effects of ethacrynic acid.

Gap junctions in the stria vascularis of guinea pigs were studied using freeze-fracture. Nearly all junctions were associated with basal cells. They were present between basal cells and spiral ligament cells, adjacent basal cells, basal and marginal cells and basal and intermediate cells. Following administration of ethacrynic acid, gap junction morphology altered. There was a statistically significant decrease in the centre-to-centre spacing of gap junction subunits and the subunits became regularly packed. Such changes were distinct before any other gross morphological change in the stria had occurred. These morphological alterations suggest that physiological uncoupling of stria cells may occur in response to the effects of ethacrynic acid.

Degradation and resynthesis of gap junction protein in plasma membranes of regenerating liver after partial hepatectomy or cholestasis.

Changes in the total amount of the gap junction protein (M(r) 26,000) after partial hepatectomy or bile duct ligation and recanalization were investigated in rat liver membranes by quantitative immunoblot with rabbit antiserum to the M(r) 26,000 protein. The loss and reappearance of the M(r) 26,000 protein roughly paralleled loss and reappearance of gap junction plaques analyzed previously under similar physiological conditions by freeze-fracture of hepatocyte surfaces. The total amount of the hepatic M(r) 26,000 protein in liver plasma membranes and the total area of the hepatocyte surface occupied by gap junction plaques appeared to be proportional under these conditions. However, at the minimum, 28-35 hr after partial hepatectomy we still find about 15% of the M(r) 26,000 protein, in contrast to <1% of gap junction plaques, determined by morphometric analysis. This discrepancy is probably due to the fact that very small gap junction plaques, single connexons, and free M(r) 26,000 gap junction subunits are missed by the morphometric analysis. At the times of the minimal amount of the M(r) 26,000 protein in hepatic plasma membranes after partial hepatectomy or bile duct ligation we found that crude hepatic lysosomal membranes of these rats contained less M(r) 26,000 protein than lysosomal membranes of nonoperated control animals. Thus, we conclude that the decrease and increase of the total amount of the M(r) 26,000 protein cannot be explained only by dispersal and reuse of gap junction subunits but are largely due to degradation and resynthesis of the M(r) 26,000 protein. No significant change in the amount of the M(r) 21,000 protein that had been isolated with gap junction plaques was observed in liver plasma membranes after partial hepatectomy. This confirms our previous conclusion that the M(r) 26,000 and M(r) 21,000 proteins are independent of each other.

Hormonal regulation of gap junction differentiation.

Thin-section, tracer, and freeze-cleave experiments on hypophysectomized Rana pipiens larvae reveal that gap junctions form between differentiating ependymoglial cells in response to thyroid hormone. These junctions assemble in large particle-free areas of the plasma membrane known as formation plaques. Between 20 and 40 h after hormone application, formation plaque area increases approximately 26-fold while gap junction area rises about 20-fold. The differentiation of these junctions requires the synthesis of new protein and probably RNA as well. On the basis of inhibitor experiments, it can be reported that formation plaques develop at about 16-20 h after hormone treatment and stages in the construction of gap junctions appear 4-8 h later. These studies suggest that gap junction subunits are synthesized and inserted into formation plaque membrane during the differentiation of the anuran ependymoglial cells.